1. Field of the Invention
This invention relates to a latex based on synthetic cis-1,4-polyisoprene rubber (hereinafter abbreviated as IR). More particularly, the invention relates to an IR-type latex which displays substantially improved wet gel strength and dry film strength and which has excellent stability and processability.
2. Description of the Prior Art
It is well known in the art that an IR latex is obtained when an IR solution prepared by solution polymerization of isoprene in the presence of a Ziegler-type catalyst or a lithium-type catalyst is first emulsified and dispersed in water with the aid of an emulsifying agent and the solvent is then removed, with or without subsequent concentration of the resultant dilute latex. See, for example, U.S. Pat. No. 3,285,869. However, the production of IR latex by methods such as the foregoing is disadvantageous from the standpoint that large amounts of polymeric coagulate tends to precipitate in the course of stripping, i.e., removal of the solvent, or in the concentration step where the dilute latex is concentrated, for example, by centrifugation. As a consequence, such an IR latex is seriously deficient in mechanical and thermal stability. The inadequate mechanical and thermal stability of IR latex is not only responsible for the difficulties experienced in the production of latex but also is a primary cause of the diverse problems encountered when the latex is subsequently processed into shaped rubber articles. For example, coagulation of the latex takes place upon the addition of a vulcanizing agent and other compounding agents under agitation as well as during the transportation of the latex.
Although the use of an emulsifying agent in substantial amounts improves the stability of a latex, it considerably detracts from the physical properties of the latex, including its wet gel strength, film-forming property and dry film strength thereof.
Irrespective of whether a substantial amount of emulsifying agent is employed or not, IR latex is inherently inferior to natural latex in wet gel strength and dry film strength. Wet gel strength is generally defined as the strength of a latex measured during coagulation, i.e., the strength of the film at the wet gelation point. Dry film strength, on the other hand, is defined as the strength of a film in which the coagulation of the latex is complete and which contains no liquid medium. Reduced levels of wet gel strength and dry film strength of a latex lead to irreversible deficiencies in the various rubber articles manufactured from the latex. For example, in the manufacture of rubber articles, such as gloves and medical and hygienic goods by a dipping process, streaks and grooves are formed on the surface of the film formed from the latex coated on the convex mold, and deformation of the film occurs when it is removed from the mold. Such problems as mentioned hereinabove often occur in the course of processing shaped articles resulting in unsatisfactory products. Furthermore, in the production of foamed rubber from such a latex, additional defects will be encountered, such as middle tear, which is attributable to inadequate wet gel strength of the latex.
As previously mentioned, IR latex has serious inherent disadvantages, i.e., deficiencies in mechanical stability which is an important factor in the preparation and molding of latex and in wet gel strength an dry film strength, both of which are important determinants of the properties of shaped articles derived from the latex and the present invention is directed to improving IR latex.